In the foundry industry, one of the processes used for making metal parts is sand casting. In sand casting, disposable foundry shapes (usually characterized as molds and cores) are made by shaping and curing a foundry mix which is a mixture of sand and an organic or inorganic binder. The binder is used to strengthen the molds and cores.
One of the processes used in sand casting for making molds and cores is the no-bake process. In this process, a foundry aggregate, binder, and liquid curing catalyst are mixed and compacted to produce a cured mold and/or core. In the no-bake process, it is important to formulate a foundry mix which will provide sufficient worktime to allow shaping. Worktime is the time between when mixing begins and when the mixture can no longer be effectively shaped to fill a mold or core.
A binder commonly used in the no-bake process is a polyurethane binder derived by curing a polyurethane-forming binder with a liquid tertiary amine catalyst. Such polyurethane-forming binders used in the no-bake process, have proven satisfactory for casting such metals as iron or steel which are normally cast at temperatures exceeding about 2500 degrees Fahrenheit. They are also useful in the casting of light-weight metals, such as aluminum, which have melting points of less than 1500 degrees Fahrenheit.
The polyurethane-forming binder usually consists of a phenolic resin component and polyisocyanate component which are mixed with sand prior to compacting and curing. Both the phenolic resin component and polyisocyanate component generally contain a substantial amount of organic solvent which can be obnoxious to smell and can create stress for the environment. Because of this, there is an interest in developing binders which do not require the use of organic solvents.
British Patent Specification 1 203 943 discloses a foundry binder which purports to be solventless, yet effective. The foundry molds and/or cores disclosed in this patent are prepared from a foundry mix consisting of a (1) a wet foundry aggregate and (2) a polyurethane-forming foundry binder comprising (a) a polyether polyol having a hydroxyl value above 200 milligrams of potassium hydroxide per gram of polyether polyol and a functionality of at least 2.5 (b) an organic polyisocyanate, and (c) an organo-metallic or amine catalyst. The foundry mix is compacted into a mold and allowed to harden until the mix has sufficient dimensional stability so that it can be removed from the mold.
The patent states that the amount of the water in the foundry aggregate is from 2 to 7% by weight based upon the weight of the foundry aggregate. The patent suggests that the water in the aggregate may contribute advantageously to an earlier attainment of dimensional stability in the foundry shape. It also states that excess polyisocyanate is desirable to compensate for the presence of water. The examples show an isocyanate index of 130.
The patent is otherwise rather indefinite. For instance the amount of the catalyst level is not given in any of the examples.